Poorly flowering hibiscus? Problem is probably fertilizer

This hibiscus has robust blooms, but if it were overfertilized, the plant would likely exhibit lush, green leaves and reduced flowering.

This hibiscus has robust blooms, but if it were overfertilized, the plant would likely exhibit lush, green leaves and reduced flowering. (Chicago Botanic Garden)

Tim JohnsonChicago Tribune

My tropical hibiscus that I overwinter inside and keep on my patio every summer has flowered poorly this year. The plant is mostly in full sun but does get some shade during the day. All summer, I fertilized once a week to promote more flowers. The plant has grown a lot with lush green leaves. What can I do with this plant to increase flowering?

— Lisa, Elmwood Park

You have likely been overfertilizing your hibiscus this year. This plant will perform best in full sun, so if shade is increasing in your garden, that can have an impact on flowering too. Plants that are grown in containers during the summer will generally need some supplemental fertilizer to perform properly. The regular watering required to keep container-grown plants alive will flush nutrients out of the growing medium. Many commercial growing media sold by the bag at garden centers will contain some nutrients for plants, but these will be depleted over time by watering and plant use.

If you repotted your hibiscus this past spring, you would have introduced some plant nutrients. Annual plantings in the ground and flowering plants like your hibiscus will respond to excessive amounts of nitrogen from fertilizer by growing larger than normal with lush, green leaves and reduced flowering.

Go ahead and stop fertilizing the hibiscus now, but do not expect any increased flowering before you need to bring it inside for the winter — the first frost is less than a month away. Treat the plant as you have in past years by cutting it back as needed to fit in the house and moving it inside.These plants need five to six hours of full sunlight a day to perform well while inside.

Since the hibiscus is tropical and does poorly in cold, rainy weather, it is best to bring it inside before nighttime temperatures drop to 40 degrees. A light freeze over a couple of nights can kill these plants. Keep your plant in a relatively small pot, since hibiscus flowers best when its roots are crowded.

Try giving your plant a rest for a few months when you bring it in by minimizing pruning, reducing watering and moving it to a cool room with bright light. Do not push it to flower over winter. It is likely that some leaves will turn yellow and fall off as the plant adjusts to a new environment inside. In March, cut the stems back, move the plant to a sunny location and increase watering. Normally, you would fertilize once new growth appears, but I would skip this step next year.

Gradually expose your plant to the outdoors in spring when nighttime temperatures are staying above 55 degrees. Start the plant in shade and increase the time in full sun over a period of seven to 10 days. The plant may lose some leaves as it adjusts to the outside environment. The hibiscus flowers on new wood, so your plant should eventually start to bloom as the new growth matures. Apply a slow-release fertilizer when you move the plant outdoors for the season.

Then monitor plant growth and flowering — you may need to add supplemental liquid fertilizer two to three times over the course of the summer. Stunted growth and off-color leaves are signs that the plant is "hungry." Plants that are starved for nutrients will also flower poorly, but the foliage tends to be an off-green color versus the lush, vigorous and dark green leaves present when it is overfertilized.

Tim Johnson is director of horticulture for the Chicago Botanic Garden in Glencoe.

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